1. Technical Field
The present invention relates to a solid-state image capturing apparatus, a driving method thereof and an electronic apparatus. In particular, the invention relates to a solid-state image capturing apparatus, such as a CMOS image sensor, having an electronic shutter function, a driving method thereof, and an electronic apparatus having the same.
2. Description of the Related Art
Most CMOS image sensors, which are a kind of a solid-state image capturing apparatus, for example, an X-Y address type solid-state image capturing apparatus, have an electronic shutter function for electronically performing an operation of resetting signal charges accumulated in photoelectric converters (light receiving units) and newly starting accumulation of signal charges. In general, a CMOS image sensor employs a so-called rolling shutter (also referred to as a focal plane shutter) scheme for performing the starting and ending of exposure with respect to each pixel low through sequential scanning.
In the case of employing a rolling shutter scheme for an electronic shutter, since an exposure period is not synchronized with respect to each pixel row, if an object being moved or an object being turned on and off is photographed, an image may be distorted or broken. In this regard, global exposure is realized to perform the starting and ending of exposure at the same timing with respect to all pixels, so that distortion can be prevented when the object being moved or the object being turned on and off is photographed.
Therefore, in order to realize the global exposure by using a CMOS image sensor, a configuration is employed, in which a holding unit (FD or CCD) for holding signal charges is provided separately from a photoelectric conversion unit. Further, the signal charges are transferred from the photoelectric conversion unit to the holding unit with respect to all pixels, so that the exposure end is simultaneously performed with respect to all pixels and synchronization of the exposure period is realized.
However, in a CMOS image sensor employing the configuration in which the holding unit of signal charges is added, during reading of signal charges (photoelectric charges) after exposure, that is, during the period for which signal charges are continuously held in the holding unit, accumulation of electric charges occurs again because the photoelectric conversion unit continues to receive incident light. Thus, when the incident light is strong, the photoelectric conversion unit may be saturated again. Further, if signal charges overflowing from the photoelectric conversion unit leak in the holding unit, an image may be broken.
Therefore, in the related art, separately from a transfer transistor that transfers signal charges in the photoelectric conversion unit to an FD (floating diffusion) section, a charge discharging transistor, which discharges the signal charges in the photoelectric conversion unit to a part different from the FD section, is provided inside the pixel (e.g., refer to Japanese Unexamined Patent Application Publication No. 2004-140149).